1. Field of the Invention
This invention relates to an alignment method suitable for an exposure apparatus of the step-and-repeat type for manufacturing semiconductive devices or an apparatus for successively effecting inspections by the step-and-repeat system, and in particular to a method of effecting precise alignment of a mask or a reticle which provides a negative for exposure and a semiconductive wafer or the like which is an object to be exposed.
2. Description of the Prior Art
In recent years, the tendency of semiconductive devices such as IC and LSI toward minuteness and higher density has rapidly progressed, and higher accuracy has been required of apparatuses for manufacturing the semiconductive devices, particularly, exposure apparatuses in which the circuit pattern of a mask or a reticle is superposedly transferred onto a circuit pattern formed on a semiconductor wafer. It is required that the circuit pattern of the mask and the circuit pattern on the wafer be superposed one upon the other, for example, with accuracy within 0.1 .mu.m and therefore, nowadays, apparatuses of the so-called step-and-repeat type in which the circuit pattern of a mask is exposed to a localized area (for example, an area corresponding to a chip) on a wafer, whereafter the wafer is caused to effect stepping by a predetermined distance and then the circuit pattern of the mask is again exposed, particularly, reduction projection type exposure apparatuses (steppers), have become the main current of the exposure apparatuses of this kind. In this step-and-repeat system, the wafer is placed on a two-dimensionally movable stage and positioned relative to the projected image of the circuit pattern of the mask and therefore, the projected image and each chip on the wafer can be precisely superposed one upon the other. Also, in the case of the reduction projection type exposure apparatus, there are two methods, namely, the through-the-lens type alignment method whereby alignment marks provided on a mask or a reticle and marks attendant to the chips on a wafer are directly observed or detected through a projection lens to thereby accomplish alignment, and the off-axis type alignment method whereby alignment of an entire wafer is effected by the use of an alignment microscope provided at a predetermined distance from a projection lens, whereafter the wafer is fed to just beneath the projection lens. Generally, the through-the-lens system has a merit that the super-position accuracy is high because alignment is effected for each chip on the wafer, but suffers from a problem that the exposure processing time of a wafer is long. In the case of the off-axis system, once alignment of the entire wafer has been completed, the wafer need only be caused to effect stepping in accordance with the arrangement of the chips and therefore, the exposure processing time is shortened. However, the alignment of each chip is not effected and therefore, satisfactory superposition accuracy has not always been obtained under the influence of the expansion and contraction of the wafer, the rotation error of the wafer on the stage, the degree of orthogonality of movement of the stage itself, etc.